This disclosure generally relates to process and apparatus for testing composite material. More specifically, the invention relates to processes and apparatus for obtaining properties of composite material under biaxial loading.
Oxide/oxide ceramic matrix composites (CMC) are being considered for use in commercial aircraft engine exhaust washed structures. Biaxial loading is inherent in acoustic, jet engine, exhaust structures when sandwich geometries are employed. There is a need for a test method and fixture design to characterize CMC materials under biaxial loading. There are likely other materials and applications (e.g., polymer composites and pressure vessels) that could be cost effectively characterized with such a test method.
Biaxial testing has been historically difficult to perform successfully. One, current test method for biaxial testing involves a cruciform coupon loaded simultaneously by two perpendicular load cells. This method requires expensive test machines and test coupons, yields unreliable results, and is not applicable to ceramic matrix composites. This method is conducted on polymeric matrix composite coupons with mixed success. Typically ceramic matrix composites fail prematurely in the grips or at the cruciform intersection, resulting in invalid data.
Another method is to load a cylindrical tube axially while pressurizing the interior. This is a complicated and expensive method with limited applicability. For instance, each test article might need to be a 10-inch-long, 10-inch-diameter cylinder, which would be complicated and expensive to make.
A test method is needed to obtain the open hole compression strength for CMC laminates under biaxial loading conditions to enable use of representative strength and failure criteria on applicable CMC structures.